One method for selecting a number of cylinders to carry out combustion is described in U.S. Pat. No. 5,479,898. In this system, engine output is reduced via ignition timing, air-fuel ratio, and cylinder deactivation in a systematic approach. However, the inventors herein have recognized a disadvantage with such a system when using a new system configuration.
In particular, the inventors herein have recognized that operating with some cylinders combusting a lean air-fuel mixture and others inducting air without injected fuel can provide significantly increased fuel economy. Further, if an emission control device (that retains NOx during lean operation and later releases and reduces the NOx during rich operation) is coupled to the combusting cylinders, then such operation can be achieved without degraded emissions.
However, the inventors herein have also recognized that such operation cannot produce the required engine output in all driving conditions. And therefore, such a mode should be selectively actuated. However, the method according to U.S. Pat. No. 5,479,898 only allows such a mode to reduce engine output below that which can be obtained with the throttle substantially closed. Since the inventors herein do not simply wish to reduce engine output, but rather desire to operate with the throttle substantially open to obtain fuel economy benefits, the prior art method of U.S. Pat. No. 5,479,898 fails to meet their requirements.